Water and Electrolytes 11 



concentration is raised due, for example, to an environment rich 

 in potassium, intracellular sodium tends to fall. This could be ex- 

 plained on the assumption that there is a limited degree of cross 

 specificity with regard to sites binding both ions. For a variety of 

 reasons, however, it is generally preferred to postulate separate 

 sodium-rich and potassium-rich zones within the cell. In particular, 

 the facts suggest at present that the sodium-rich zone is at the peri- 

 phery of the cell (in the sense of being more accessible to isotopic 

 exchange) and consists perhaps of an annulus about 3 [x thick. 



Thus this current concept of the cell postulates an ion-binding 

 matrix, part specific for potassium, part accessible to other ions 

 including sodium, the sodium zone at the "periphery," the potassi- 

 um at the "centre," the potassium binding sites being inaccessible 

 to sodium provided that the cell retains its integrity. It is also likely 

 that although potassium is present in the interstices, the binding 

 sites in the sodium zone are inaccessible to potassium ion in the in- 

 tact cell. 



The substitution of ion exchange within the cell for the classical 

 view of electrolyte metabolism is compatible with the facts pre- 

 sented above. The theory is also compatible with observations con- 

 cerning electrolyte movements in injury. It is necessary to assume 

 that the specific ion-binding properties of the matrix, particularly 

 for potassium are dependent on the integrity of this structure which 

 in turn depends ultimately but not exclusively upon metabolic re- 

 actions and that in the early stages of injury, such a loss of specificity 

 is reversible. Thus the exchange of extracellular potassium with 

 potassium loosely bound to the matrix would not of itself require 

 energy; the exclusion of ions other than potassium from the ion- 

 binding sites might require active metabolism, if only to maintain 

 the structural integrity of the ion-binding system. 



There is good evidence that discrepancies may exist between 

 the metabolic integrity of cells and their ability to maintain normal 

 electrolyte gradients. Thus strophanthin and tetrabutylammonium 

 affect ionic movements without apparently having the expected 

 adverse actions on cell metabolism (acetyl choline and adrenaline 

 have similar properties) . Harris has suggested that in injury of 

 the strophanthin type there is not so much loss of potassium-binding 



